Session B17: Graphene: Synthesis, Properties, and Defects

Synthesis and Properties of 2D Atomic Layers: from Graphene to 2D-GaN

Beyond graphene, there is a huge variety of layered materials that range in properties from insulating to superconducting. Furthermore, heterogeneous stacking of 2D materials also allows for additional ``dimensionality'' for band structure engineering. In this talk, I will discuss recent breakthroughs in two-dimensional atomic layer synthesis and properties, including novel 2D heterostructures and novel 2D nitrides. Our recent works include development of an understanding of substrate impact on 2D layer growth and properties, doping of 2D materials with magentic elements, selective area synthesis of 2D materials, and the first demonstration of 2D gallium nitride (2D-GaN). Our work and the work of our collaborators has lead to a better understanding of how substrate not only impacts 2D crystal quality, but also doping efficiency in 2D materials, and stabalization of nitrides at their quantum limit.   

Line defects in Graphene: How doping cures the electronic and mechanical properties

Graphene and carbon nanotubes have extraordinary mechanical properties. Intrinsic line defects such as local non-hexagonal reconstructions or grain boundaries, however, significantly reduce the tensile strength and destroy its unique electronic properties. Here, we address the properties of line defects in graphene from first-principles on the level of full-potential density functional theory, and assess doping as one strategy to strengthen such materials. We carefully disentangle the global and local effect of doping by comparing results from the virtual crystal approximation with those from local substitution of chemical species, in order to gain a detailed understanding of the breaking and stabilization mechanisms. We find that n-type doping or local substitution with electron rich species increases the ultimate tensile strength significantly. In particular, it can stabilize the defects beyond the ultimate tensile strength of the pristine material. We therefore propose that this should be a key strategy to strengthen graphenic materials. We find that doping can furthermore lead to semi-conducting behaviour along line defects, ultimately restoring the unique electronic properties of graphene.   

Polycylcic carbon molecules with zigzag edges as sources of defects in graphene on a metal

Unlike the armchair edge, the zigzag edge of graphene breaks the equivalence of its two constituting carbon sub-lattices. Uncompensated magnetic moments are thus expected for such edges. For the same reason, dense polycyclic molecules (PCMs) terminated by zigzag edges are predicted to host net magnetic moments. Unfortunately, their synthesis is challenging. One approach relies on the pyrolysis of hydrocarbons, catalyzed by a transition metal. Here we investigate this little-explored approach, and put in evidence the formation of a series of highly symmetric zigzag edge PCMs onto Re(0001), among which phenalene, coronene and sumanene. We also address the relationship between the preparation of such molecules and graphene, which both form following hydrocarbon pyrolysis. We establish that the PCMs are unexpected obstacles towards high quality graphene.   

Electronic Transport through linear strain defects in Graphene

Strain­-induced pseudo magnetic fields in Graphene have been studied by STM [1] as well as theoretically.[2] Such pseudo magnetic fields can confine electrons by the presence of magnetic barriers [3] or by the formation of closed cyclotron orbits. Here we report transport measurements through a nanometer­-scale width, but micron-­scale length linear strain defect in a graphene sheet. The transport data exhibits Coulomb blockade features, indicating the presence of a quantum dot. The charging energy and level spacing are consistent with the defect forming a one-dimensional quantum wire, similar to a carbon nanotube. This suggests the possibility that such defects can be used to confine or guide electrons in graphene. Our latest results will be discussed. [1].Strain? Induced Pseudo–Magnetic Fields Greater Than 300 Tesla in Graphene Nanobubbles ,N. Levy ,M. F. Crommie etc, Science [2].?Energy gaps and a zero? field quantum Hall effect in graphene by strain engineering, F. Guinea, A.Geim , Nature Physics [3].Magnetic Confinement of Massless Dirac Fermions in Graphene,A. De Martino, L. Dell’Anna, and R. Egger, Physical Review Letters   

The Effect of Defects on Mechanical Properties and Failure Mechanisms of Graphene

Recent experiments involving nanoindentation of graphene have demonstrated counterintuitive increasing of Young's modulus with increasing concentrations of point defects in graphene. To fully resolve this controversy we perform large-scale molecular dynamics simulations of graphene nanoindentation. The relaible description of interatomic interactions is achieved by using recently developed screened environment-dependent bond order (SED-REBO) potential. The elastic properties of the defective graphene, the breaking strength and the mechanisms of fracture under indenter are investigated as a function of defect concentration and other factors specific to Atomic Force Microscopy (AFM) nanoindentation experiments.   

Mechanical Behavior of Graphene Nanomeshes

Graphene nanomeshes (GNMs) are ordered, defect-engineered graphene nanostructures consisting of periodic arrays of nanopores in the graphene lattice with neck widths less than 10 nm. The electronic, transport, and mechanical properties of GNMs can be tuned by varying the structural, chemical, and architectural parameters of the nanomeshes, namely, their porosity, as well their pore lattice structure, pore morphology, and pore edge passivation. Here, we study the mechanical response of GNMs to uniaxial tensile straining and determine their mechanical properties based on molecular-dynamics simulations of dynamic deformation tests according to a reliable bond-order interatomic potential. We establish the dependences of the elastic modulus, fracture strain, ultimate tensile strength, and toughness on the nanomesh porosity and derive scaling laws for GNM modulus-density and strength-density relations. We also establish the dependence of the above properties on pore morphology, for GNMs with circular and elliptical pores over a range of aspect ratios, and on pore edge hydrogen passivation that causes elastic stiffening and strength reduction. The underlying mechanisms of crack initiation and propagation and nanomesh failure also are characterized.   

Fe-catalyzed etching of graphene layers

We investigate the Fe-catalyzed etching of graphene layers in forming gas. Fe thin films are deposited by sputtering onto mechanically exfoliated graphene, few-layer graphene (FLG), and graphite flakes on a Si/SiO2 substrate. When the sample is rapidly annealed in forming gas, particles are produced due to the dewetting of the Fe thin film and those particles catalyze the etching of graphene layers. Monolayer graphene and FLG regions are severely damaged and that the particles catalytically etch channels in graphite. No etching is observed on graphite for the Fe thin film annealed in nitrogen. The critical role of hydrogen indicates that this graphite etching process is catalyzed by Fe particles through the carbon hydrogenation reaction. By comparing with the etched monolayer and FLG observed for the Fe film annealed in nitrogen, our Raman spectroscopy measurements identify that, in forming gas, the catalytic etching of monolayer and FLG is through carbon hydrogenation. During this process, Fe particles are catalytically active in the dissociation of hydrogen into hydrogen atoms and in the production of hydrogenated amorphous carbon through hydrogen spillover.   

The Production of Graphene Coated Surface by PVD and Investigation of Its Electrical and Optical Properties

Graphene is used for a broad range of applications with unique properties such as lightweight, flexibility, mechanical strength, best electrical and thermal conductivity in industrial processes like electronics,medicine,energy,sensors, and other areas related with material usage variations. There are a few common methods to produce graphene. In this study PVD method was used to produce graphene. Many challinging issue was handled in this method like production temperature, growth surface, and mechanical effects. Produced graphene was studied by XRD, scanning electron microscopy (SEM), Raman spectroscopy, and AFM.   

Defects, Strain, Incommensurability and Polymorphism in Graphene on Metals

The prevalence of defects in large-area graphene fabricated on metal substrates may undermine the unique properties that are vital to its use in technological applications. Although efforts to mitigate these imperfections have met with some success, they may alternatively be harnessed to tailor graphene's properties or alter its functionality. We have studied the growth/defect structure of graphene/metals using low energy electron microscopy (LEEM) and micro-low energy electron diffraction ($\mu $-LEED). These investigations reveal the proliferation of small-angle lattice orientational disorder and small angle grain boundaries in graphene/Ru(0001) prepared by conventional ethylene CVD at high temperature. Although orientationally uniform graphene could be produced by a hybrid CVD/segregation method, this layer exhibits significant incommensurability and polymorphism, i.e. several commensurate structures. Two-dimensional strain mapping in graphene/Ir(111) obtained from scanning $\mu $-LEED measurements using a 250nm probe beam reveals inhomogeneous strain relaxation by wrinkles. This suggests that it may be possible to strain engineer the properties of graphene if wrinkling can be controlled to form desirable wrinkle networks . Coupling of lattice rotation and strain is also observed by the same approach in graphene on other metal substrates.   

\textbf{A single}\textbf{-}\textbf{step growth process of graphane using hydrogen plasma and observation of an induced bandgap.}

There has been considerable interest in reliably opening up a bandgap in graphene for electronic applications. One promising method is the hydrogenation of graphene into graphane. We present Raman spectroscopy, scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray photoemission spectroscopy (XPS) studies of hydrogenated multilayer graphene on Cu as a function of hydrogen exposure time ($t)$. Our growth process for hydrogenated graphene involved \textit{in-situ} exposure of PECVD-grown graphene on Cu to hydrogen plasma. Raman measurements revealed an increase in intensity of a pronounced and narrow D-band with $t$ when compared to pristine graphene. FTIR studies revealed the presence of C-H bonds on the surface of our samples post hydrogenation. STM topographic studies revealed a nanoscale Moir\'{e} pattern resulting from the hydrogenated graphene. For $t \quad =$ 120s, STS studies revealed an average gap of $\Delta $ \textasciitilde (0.275\textpm 0.050) eV, which increased to average value of $\Delta $ \textasciitilde (0.315\textpm 0.050) eV for $t \quad =$ 600s. Topographic and spectroscopic studies showed approximate hydrogen coverage of 20{\%}, 50{\%} and 80{\%} for $t \quad =$ 30s, 60s and 120s, respectively. XPS studies of the C-1s state revealed an energy shift from the C-C peak (284.6 nm) towards a C-H peak (285.8 nm), consistent with the formation of carbon-hydrogen bonds. Our results have demonstrated the existence of a bandgap opening in graphene, induced by the adsorption of atomic hydrogen onto graphene.   

Ion irradiation of graphene on Ir(111): From trapping to blistering

Graphene grown epitaxially on Ir(111) is irradiated with low energy noble gas ions and the processes induced by atomic collision and subsequent annealing are analyzed using scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron diffraction and thermal desorption spectroscopy. Upon room temperature ion irradiation graphene amorphizes and recovers its crystalline structure during annealing. The energetic noble gas projectiles are trapped with surprisingly high efficiency under the graphene cover up to extremely high temperatures beyond 1300K. The energy, angle, and ion species dependence of trapping are quantified. At elevated temperatures the trapped gas forms well developed and highly pressurized blisters under the graphene cover [1-3]. We use molecular dynamics simulations and ab initio calculations to elucidate the trapping mechanism and its thermal robustness. Similar trapping and blistering are observed after ion irradiation of a single layer of hexagonal boron nitride on Ir(111) and we speculate on the generality of the observed phenomena. [1] C. Herbig et al., ACS Nano 8, 12208 (2014). [2] C. Herbig et al., ACS Nano 9, 4664 (2015). [3] C. Herbig et al., PRB 92, 085429 (2015).   

Visualizing Klein tunneling in graphene at the atomic limit

Graphene has attracted much attention from both the solid-state and high-energy scientific communities because its elementary excitations mimic relativistic chiral fermions. This has allowed graphene to act as a table-top testbed for verifying certain longstanding theoretical predictions dating back to the very first formulation of relativistic quantum mechanics. One such prediction is Klein tunneling, the ability of chiral electrons to transmit perfectly through arbitrarily high potential barriers. Previous transport and point-spectroscopic studies confirmed Klein behavior of graphene electrons but lacked real-space resolution. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) measurements to present the first real-space atomic images of Klein tunneling in graphene. In these CVD-grown samples, quasi-circular regions of the copper substrate underneath graphene act as potential barriers that can scatter and transmit electrons. At certain energies, the relativistic chiral fermions that Klein scatter from these barriers are shown to fulfill resonance conditions such that the transmitted electrons become trapped and form standing waves. These resonant modes are visualized with detailed spectroscopic images with atomic resolution that agree well with theoretical calculations. The trapping time is shown to depend critically on both the angular momenta quantum number of the resonant state and the radius of the trapping potential.